The field of the present invention is hypodermic syringes. The invention has particular application to those hypodermic syringes which are used for administering medicaments such as vaccines, local anesthetics and antibiotics which are designed to be injected intramuscularly, subcutaneously, or intradermally but not directly into a vein or artery. The invention also has particular application to those hypodermic syringes which are used in introducing catheters and tubes into the lumens of blood vessels. The present invention is directed to certain improvements in the construction of a hypodermic syringe which facilitate one-handed performance of an aspiration test, hereinafter further described, which is useful to avoid the accidental injection of medicaments into blood vessels or else to confirm that the tip of the needle is in the lumen of a blood vessel when this is the desired goal, as in catheterizing a blood vessel. The improvements are ones which are suitable for incorporation into a fully disposable, single-use syringe of compact size and configuration which has barrel and plunger components that can be made of moldable plastics.
Correct technique for injecting medicaments which are intended for hypodermic but not intravenous injection requires the practitioner to insert the syringe needle into muscle, subcutaneous or intradermal tissue and then draw back on the plunger and observe whether blood is or is not aspirated into the syringe barrel. If the practitioner observes blood being aspirated, the practitioner takes this as an indication that the needle has perforated a vein or artery, and the needle is then withdrawn and the injection procedure started over at a different site. Performance of this so-called aspiration test is recommended in numerous guides and treatises which physicians and other clinical professionals regularly consult. For example, a well-known nursing training text includes this instruction: "Aspirate by holding the barrel of the syringe steady with your nondominant hand and by pulling back on the plunger with your dominant hand. If blood appears in the syringe, withdraw the needle, discard the syringe, and prepare a new injection. Rationale: This step determines whether the needle is in a blood vessel." (B. Kozier & G. Erb, Fundamentals of Nursing Concepts and Procedures 3d Ed., Addison-Wesley 1987!, p. 1521.) Another well-known nursing treatise, in instructing on how to administer a subcutaneous injection, recommends the aspiration step "to determine whether the needle is in a blood vessel" and warns that "d!iscomfort and possibly a serious reaction may occur if a drug intended for subcutaneous use is injected into a vein." (L. Wolf, M. H. Weitzel & E. Fuerst, Fundamentals of Nursing 6th Ed., J. B. Lippincott 1979!, p. 614). Similarly, a Physicians' Desk Reference (hereinafter cited simply as "PDR") article on Haemophilus b Conjugate (Tetanus Toxoid Conjugate) reconstituted with CLI DTP (Diphtheria, Tetanus and Pertussis) vaccine warns that the vaccine is not to be injected intravenously and recommends: "After insertion of the needle, aspirate to ensure that the needle has not entered a blood vessel." (R. Arky, medical consultant, Physicians' Desk Reference 49th Ed., Medical Economics Data Prod. Co. 1995!, p. 903.) In addition to the cited references, there are numerous others which recommend that the aspiration test be performed to minimize risk of unintentionally injecting medicaments into blood vessels.
It is noted in the medical literature that failure to perform the aspiration test, resulting in the accidental injection into a blood vessel of a medicament not intended for that mode of administration, may have serious consequences for the patient. For example, in L. Tessio, L. Bassi and L. Strada, Spinal Cord Lesion After Penicillin Gluteal Injection, 6 Paraplegia 442 (June 1992), the authors state that in eight cases seen by or reported to them, gluteal injection of penicillin caused sudden and irreversible paraplegia. The authors suggest that "the mechanism might be the accidental injection into the superior gluteal artery, causing its distal spasm and the upstream ascent of the penicillin with ensuing embolic and/or spastic occlusion of the anterior spinal artery." For another example, it is noted in J. R. Roberts & J. R. Hedges, Clinical Procedures in Emergency Medicine Saunders 1985!, p. 436, that there is risk of inducing convulsions and generalized seizures in patients by injecting local anesthetics (intended for intramuscular or subcutaneous administration) into an artery by mistake.
As for vaccines, there is apparently little or no published evidence of toxicity to the patient resulting from accidental injection into a blood vessel. However, for absorption reasons, vaccines are typically designed to be injected intramuscularly or subcutaneously, and immunogenicity from vaccines can depend on proper administration. Hence, there is at least reason for concern that a vaccine intended for intramuscular or subcutaneous injection might not have the same efficacy if injected into a vein or artery, even if no toxicity to the patient would thereby result. (In addition to the PDR article cited above regarding the Haemophilus b Conjugate (Tetanus Toxoid Conjugate) reconstituted with CLI DTP (Diphtheria, Tetanus and Pertussis) vaccine, PDR articles pertaining to other vaccines as well caution that they are not to be injected intravenously. See, e.g., articles on the Measles, Mumps and Rubella Virus Vaccine, PDR, p. 1575, and on the Hepatitis B Vaccine, id., p. 2372).)
Although reports of adverse effects on patients from accidental injections of medicaments into blood vessels are apparently not common, the severity of possible risks dictates that the aspiration test should be performed routinely, whenever an intramuscular, subcutaneous or intradermal injection of a medicament is administered.
In most clinical settings involving normal adult patients, the drawing-back-the-plunger maneuver, and hence the aspiration test, can be accomplished without any intolerable amount of difficulty because the patient can ordinarily be relied upon to hold still voluntarily. The practitioner thus has both hands free, and so can grasp the barrel of the syringe in one hand while drawing back the plunger with the other hand to check for aspiration of blood. Even with normal adults, however, there are clinical situations (such as, for example, the injection of a local anesthetic into a painful area) in which the need to use one hand to immobilize the injection site, leaving the practitioner with only one hand free to operate the syringe, poses serious inconvenience to a practitioner trying to perform the aspiration test.
Where the patient is a child, performance of the drawing-back-the-plunger maneuver is especially likely to prove cumbersome and inconvenient with a conventional syringe. Children, in contrast to normal adults, often struggle when receiving injections, and cannot be depended upon to possess the self-control necessary to hold still voluntarily. Hence, a practitioner who is, for example, injecting a medicament into the arm of a child must take steps to see that the child's arm is held immobile during the injection. Otherwise there is risk of such mishaps as the practitioner's suffering an accidental needle stick, or breaking off the needle in the patient's arm, or accidentally withdrawing the entire needle, or lacerating the patient's tissue. Unless the practitioner has an assistant to hold the child, the practitioner must typically hold the child's arm immobile with one hand while operating the syringe with the other hand. The practitioner therefore does not have two hands free to accomplish the drawing-back-the plunger maneuver; and with a syringe of the kind now conventionally used for the intramuscular, subcutaneous or intradermal injection of vaccines, antibiotics and other medicaments, the maneuver is very difficult, if not altogether impossible, to perform with one hand. The result is that the practitioner is tempted to, and all too often does, forego the drawing-back-the-plunger maneuver, omit the aspiration test, and hence fail to make certain that a vein or artery has not been perforated by accident. Correct technique, and its attendant advantages in terms of safety and efficacy, are thereby sacrificed for convenience's sake.
There are clinical situations where the practitioner, rather than desiring to avoid perforating a blood vessel, desires instead to confirm that such perforation has been accomplished. One example of such a situation is when the practitioner desires to accomplish a venous catheterization, that is, the introduction of a catheter, or tube, into the lumen of a vein. Such catheterization may be desired, for example, in order to secure intravenous access for delivery of medicaments or in order to guide a sensing device. to the patient's heart to take cardiac measurements. Typically, catheterization is accomplished by first inserting a relatively large-bore hypodermic needle through a vein, often without a syringe attached. When the practitioner believes that the needle has achieved the through-the-vein position, a syringe containing saline is attached and, while the practitioner withdraws the plunger in an aspirating maneuver, he or she simultaneously begins a gradual withdrawal of the syringe and needle. The practitioner looks for aspirated blood to appear in the syringe, indicating that the tip of the needle has now been withdrawn to the point where it is in the lumen of the vein. With the needle so positioned, the practitioner detaches the syringe and introduces a guidewire into the vein through the bore of the needle. The needle is then slipped out over the guidewire and a catheter is slipped in over the guidewire. Once the catheter is in place the guidewire is withdrawn through the catheter. A syringe is then attached to the catheter and an aspiration stroke is typically performed, the appearance of aspirated blood in the syringe being a reconfirmation that the catheter is correctly positioned in the lumen of the vein. The syringe typically is then used to flush the catheter with heparin to inhibit formation of clots and emboli.
The venous catheterization technique just described depends, of course, on the practitioner's success in positioning the tip of the needle within the lumen of the vein; and the aspiration-while-gradually-withdrawing-the-needle maneuver is important in that it confirms that that positioning has been achieved. Syringes according to the present invention, because they render it ergonomically easier than conventional syringes to accomplish the aspiration stroke while handling the syringe with a single hand, enhance the ease and convenience with which the practitioner can accomplish the aspiration-while-gradually-withdrawing-the-needle maneuver.
So-called "control syringes" are known in the art which include a pair of finger rings affixed to opposite sides of the barrel (the barrel rings), together with a thumb ring atop the plunger. By inserting the index and middle fingers through the barrel rings and the thumb through the thumb ring, the practitioner can achieve a measure of control over the syringe and can operate the plunger on the backward (withdrawing) stroke with one hand. However, the three-rings arrangement of the "control syringe" is typically seen in a syringe much larger than syringes designed for the intramuscular injection of medicaments and syringes designed for use in venous catheterization procedures. "Control syringes" are typically not intended for those uses, but instead for irrigation and the like.
It would not be immediately apparent or obvious to adapt the three-rings feature of the "control syringe" to provide a one-hand-operable syringe for the intramuscular injection of medicaments, because the resulting syringe would have substantial obvious disadvantages. First, the barrel rings, being relatively large and bulky, would increase material cost, would increase the complexity of molding the syringe body if it were attempted to mold it in a single piece, and would make the syringe less compact to package, ship and store. While making the syringe barrel itself larger might make it easier to attach and configure the rings, the use of an unnecessarily large disposable syringe would make for unnecessary cost and waste of materials, and could also make difficult the accurate measurement of doses as small as some doses, especially pediatric doses, can be (e.g., 0.5 cc or the like). Thus, syringe designers would not consider such a design to be acceptable. Secondly, the manual maneuver required to insert the fingers through the barrel rings would be slow and awkward compared to the simpler maneuver of grasping the barrel of the syringe between the fingers (or between finger and thumb) in the region of the finger-stops of the present invention as hereinafter described. Thirdly, neither the barrel rings nor the thumb ring on a conventional "control syringe" are adjustable, but are instead made in fixed diameters designed to admit the largest fingers and thumbs expected to be encountered. Practitioners with average or smaller than average sized fingers will find their grasp of a control syringe to be less than optimally secure because of the looseness, or play, which results from the excessive size of the rings. Fourthly, the non-adjustable ring atop the plunger of a control syringe, necessarily being designed to accommodate the largest normal thumb, will be much too large to make a snug fit with a small or even a normal index finger inserted therethrough; whereas the adjustable plunger loop of the present invention, as hereinafter described, affords equal convenience to those practitioners who choose to operate the plunger with the index finger while grasping the barrel between the thumb and middle finger (as is typical in administering vaccines) and those practitioners who prefer to operate the plunger with the thumb while grasping the syringe barrel between the index and middle fingers (as is typical in administering local anesthetics).